Diaphragm pumps have found wide application because of the fact that they work essentially leak free, as compared to conventional piston pumps, and do not contain parts as susceptible to wear, which may also contribute to contaminating the pumping medium. In diaphragm pumps, the diaphragm is not directly driven by a mechanical component, but rather is driven through a hydraulic pressure medium, usually oil, and hereinafter referred to as oil, which in turn is activated by a mechanical piston. This piston is not particularly susceptible to sealing problems since leakage oil, if any, may be supplied to an oil reservoir from which the oil volume between the piston and the diaphragm is automatically refilled. In this type of pump, the diaphragm forms the boundary between the pumping medium to be delivered and the working oil.
High pressure liquid chromatography (HPLC) is one of the fields of application of diaphragm pumps of the type described above. Growth in this technology has been toward increasing pressures at ever decreasing flow rates. Here, a disadvantage inherent in the design of diaphragm pumps makes itself felt, namely the pulsating flow of the liquid delivered. When a diaphragm pump is used in liquid chromatography, it is necessary that this pulsation be damped enough to ensure that it will not vitiate the analysis. To effect this damping, one generally employs damping elements containing the medium delivered which are adapted to increase their volume as the pressure rises and to reduce it again when the pressure drops. Thus, a "capacitor effect" is achieved meaning that part of the medium delivered by the pump is stored during the pressure phase and released again via a flow resistance during the other phase of the pump when its pressure drops at the high pressure end. In this manner, a certain uniformity of flow is achieved.
A damper of this type is described by Achener, U.S. Pat. No. 4,222,414, issued Sept. 16, 1980 wherein delivered fluid is caused to pass through a sealed expandable plastic tube immersed in a sealed chamber of a compressible liquid. A pressure pulse in the delivered fluid is damped by the radial expansion of the plastic tube into the compressible liquid. Bourdon tubes and compressible liquid or spring loaded diaphragms also are typical of the above described damping technique. The function of these prior art dampers is similar to that of RC elements in an electrical circuit; namely the damping behavior is a function of the pumping frequency. In addition, the equilibrium volume of the damping element increases as the absolute pressure of the delivered medium rises creating dead volume which is undesirable in modern high pressure liquid chromatography. For example, in light of the present tendency to ever smaller flow quantities and ever higher pressures, a dead volume of even 1 ml is unacceptable, since it would appreciably widen the peak in an HPLC chromatogram.
Ernst, et al., U.S. Pat. No. 3,984,315 issued Oct. 5, 1976 describes a damping device which predominatly overcomes the limited performance range of prior art RC type dampers. Here, a manually adjustable spring is coupled to the diaphragm of a diaphragm damper to provide adjustable stiffness to the damping chamber. When used at high absolute fluid pressures the spring is manually compressed to raise the effective stiffness of the diaphragm and restrict the dead volume expansion of the damper chamber. At low fluid pressures the stiffness is adjusted accordingly lower. In this manner, an appropriate balance between damping and dead volume can be adjusted for a given pump operating pressure condition. The primary shortcoming of this technique is the inconvenience of the manual set point adjustment and the fact that in liquid chromatography operating pressures are not always constant; a loss in absolute pressure would cause diminished damping, whilst a gain in absolute pressure would cause a rise in dead volume for a given spring preload.
A second disadvantage of diaphragm pumps concerns the need to provide a means for regulating the oil pressure developed between the piston and diaphragm. Once the diaphragm has reached its full deflection with hydraulic pressure, any residual stroke of the diaphragm pump piston will incur a rapid pressure increase of the oil over the diaphragm pressure which could be damaging to the diaphragm, pump seals and valves. In the prior art measures to limit excessive oil pressure development have consisted of the placement of a pressure regulating valve in the oil chamber between the piston and diaphragm to vent the high pressure oil back to the pump oil reservoir, or the placement of a preloaded spring between the piston and its drive mechanism to restrict piston displacmeent beyond an oil pressure set point. In either case, the oil override set point must be set at an oil pressure greater than the maximum downstream delivered fluid pressure to insure sufficient oil pressure to cause proper deflection of the diaphragm with each stroke of the piston over all operating conditions and delivery pressures. Since in liquid chromatography the analysis is most often obtained at average pump pressures substantially below the maximum operating point of the pump, the prior art diaphragm pump is usually substantially overworked, causing premature wear of seals, valves, and other parts in each pump.